Cation exchange resins and anion exchange resins are widely used for removing ionic species and particulates from water streams. A typical cation exchange resin includes a copolymer, such as a styrene-divinylbenzene copolymer, to which sulfonic acid anionic sites have been attached by a sulfonation reaction. A typical anion exchange resin includes a co-polymer to which ammonium salt cationic sites have been attached.
Useful cation exchange and anion exchange resins contain essentially no leachable compounds, organic or inorganic, that might leave the resin and contaminate the water stream. Additionally, the cation and anion exchange resins must possess sufficient hydraulic stability to resist compressive forces exerted by a flowing stream of water and substantially maintain their shape.
In order to provide such useful ion exchange resins, resin designers have adjusted hydration, swelling, and porosity of ion exchange resins through proper choice of polymers and copolymers and through control of the degree of cross-linking of polymer chains. Additionally, resins having pores with dimensions significantly larger than the molecular distance between adjacent copolymer chains (macroporous resins) have been synthesized. Such resins are referred to as macroporous resins, also called macroreticular resins. The terms macroporous and macroreticular are synonymous, as are the terms microporous and microreticular. Microreticular resins are also called gelular resins.
Macroporous resins have been synthesized by the inclusion of various uncross-linked polymers in a monomer mixture. The included polymers are rendered soluble and leachable following sulfonation or amination. The leachable polymers are removed, leaving relatively large-sized pores throughout the cation exchange resin or the anion exchange resin. Other macroporous resins have been synthesized by polymerizing a resin in a solvent which dissolves monomer reagents but exerts essentially no solvent action on the copolymer produced.
Cation exchange resins and anion exchange resins have been used to purify many different types of water streams, ranging from household drinking water to industrial waste. With the development of modern high-pressure steam generation systems, cation exchange resins and anion exchange resins are currently employed to treat boiler feedwater makeup streams and recycle condensate water streams. Condensate polishing units containing cation exchange resins and anion exchange resins are used to remove impurities, such as iron oxide corrosion products, alumina, and silica from condensate streams produced in chemical manufacturing processes and in electrical power generating plants, both fossil-fueled and nuclear-powered. Similarly, cation exchange resins and anion exchange resins are used to produce ultrapure water streams required in the electronics industry, for example, for producing semi-conductors.
Condensate polishing units containing cation exchange resins and anion exchange resins are the norm for modern steam generation systems operating in the range of about 900 to about 3500 psig. The use of condensate polishing units leads to improved turbine efficiency, shorter unit startup time, protection from the effects of condenser leakage, reduced radiation exposure to personnel, and longer intervals between acid cleanings. The ion exchange resins may be present in relatively deep beds or in coatings on filters.
The use of cation exchange resins and anion exchange resins to purify condensate streams in boiling water reactor (BWR) nuclear reactors is illustrative. In a BWR system, water having a pH of about 7 is circulated through a nuclear reactor core producing saturated steam which is directed to a steam turbine generator and, thereafter, passed to a condensate polishing unit. A small amount of the steam may be converted to radioactive tritium, but in many cases the steam itself is not a significant source of radioactivity. However, impurities in the steam, such as iron oxide and copper oxide particulate, are more readily converted to radioactive isotopes. Additionally, water which has been used to submerge radioactive fuel elements during storage and, subsequently, contains some radioactive particles (i.e. "low level rad waste") may be mixed into the condensate stream for clean-up.
As in fossil-fired power plants, improved turbine efficiency and protection from an accumulation of deposits in the steam systems of nuclear plants are important considerations. Consequently, ion exchange resins are employed in condensate polishing units to remove metal oxide particles which are often present in BWR condensate in the range of about several parts per billion by weight. Cation exchange resins have been developed specifically for this application. These cation exchange resins remove the insoluble iron and copper oxides by means of adsorption and filtration. In similar fashion impurities such as silica are removed from nuclear plant condensate streams by anion exchange resins.
More recently, it has been noted that some of the ion exchange resins used for purifying nuclear BWR condensate streams demonstrate better iron removal efficiency after several months of service. Experts theorize that a fraction of the divinylbenzene linkages, which cross-link styrene polymer chains in the resins, yield to oxidative attack during service. The resins with a reduced degree of cross-linking appear to remove iron from condensate more efficiently. Accordingly, cation exchange resins were developed which contain a relatively low degree of divinylbenzene cross-linking between styrene polymer chains. The cation exchange resins with relatively low degree of cross-linking have been tested in at least one condensate polishing unit serving a nuclear BWR reactor and increased iron removal efficiency has been reported. Similarly, anion exchange resins which had been in condensate polishing service for a significant period of time are reported to exhibit improved friability and rinsing characteristics.
However, the testing of new cation and anion exchange resins on a relatively large pilot plant scale is both expensive and time-consuming. Many types of conventional ion exchange resins are available to steam system operators for removing iron oxides and silica from condensates. From the myriad of products available, steam system operators attempt to select ion exchange resins that can be used to safeguard and improve steam system performance.
Unfortunately, few guide lines exist for selecting an ion exchange resin capable of removing oxides to the very low levels required for boiler feed water in high-pressure steam systems. Selecting a resin by trial and error is a task of elephantine proportions, since there are no known methods for performing such studies in the laboratory. Further, there are no known methods for preparing simulated condensates containing oxides, such as iron oxides, silica, alumina, and zinc oxides, having the correct chemical and physical properties for bench-scale laboratory testing. At the current state of the art, tests must be conducted on a relatively large pilot plant scale, usually on the site of an industrial operation.
Although conventional ion exchange resins have proven useful for condensate polishing in the past, a need exists for an anion exchange resin having an enhanced capacity for those oxides which are substantially insoluble in water at neutral and acidic conditions. Such suspended or colloidal oxides are not efficiently removed by cation exchange resins. A desirable anion exchange resin would demonstrate removal efficiencies of 90% or more and produce a product stream that contained only a few parts per billion by weight of silica and alumina, in conformance with limits set by the nuclear power generation industry. Such an anion exchange resin would adsorb as much of the acid-insoluble oxides as possible. Also, the rate of diffusion of dissociated anions into the resin structure would be high.